Maybe because high penalties + high enforcement == rebellion?
A traffic-related anecdote: the geograplically-isolated suburb I live in had a metric fuckton of speed traps about twenty years ago - like, an absurd quantity. You had cops staking out basically everywhere and giving massive tickets for the slightest infractions. Did this actually make people stop speeding? No way. It got enough people pissed enough that the entire municipal government was voted out on a program of ending the absurd overenforcement of traffic laws.
Now, it also didn't help that there was a widespread perception that the city government was using this to line its coffers, but I think the major takeaway is that citizens will only put up with so much if infractions that nobody really sees as serious are consistently, harshly punished. Eventually, you're going to get a backlash.
Of course, this calculus would probably be different if what was being punished wasn't crimes largely committed by a politically active upper middle class, but I think the example is instructive for many cases.
I guess prohibition also kinda falls into this category, so I should refine my original statement to "in cases of high enforcement/high penalties where most people don't see the big deal, those regimes will not last long via an accelerative mechanism."